1. Field of the Invention
The present invention relates to an etalon-type optical element capable of varying periodic transmission wavelength characteristics of an optical element, which is used in the field of optical communications or the like, and a wavelength characteristics variable apparatus, an optical amplifier and an optical transmission system, using the etalon-type optical element.
2. Description of the Related Art
In optical transmission systems which transmit optical signals over a long distance, there has been recently employed a wavelength division multiplexing (WDM) optical transmission system that increases the number of optical amplification repeaters for increasing long-distance transmission, and also multiplexes a plurality of optical signals having different wavelengths to transmit the multiplexed optical signals for increasing the transmission quantity. With this WDM optical transmission system, WDM optical signals are amplified together in the optical amplification repeater and transmitted. As the optical amplifier for the repeater, for example, an erbium (Er)-doped optical fiber amplifier (EDFA) is widely used.
However, gain characteristics of this EDFA have generally wavelength dependency based on the composition of a base material of optical fiber, and hence the gain characteristics do not become completely flat gain wavelength characteristics in the wavelength band of such as from 1.5 μm to 1.6 μm, which is used in the long-distance optical transmission. Therefore, when the number of optical amplification repeaters are increased, the wavelength dependency is increased, thereby causing a problem in that a deviation in signal-to-noise ratio of light (optical SNR) is increased between each wavelength (channel).
As a conventional technique for solving such a problem, there is known a technique in which, for example, an etalon-type optical element (optical filter) having transmission (or loss) wavelength characteristics opposite to the gain wavelength characteristics of the EDFA is inserted in the optical amplifier or on the optical transmission line as a gain equalizer (GEQ), to flatten the gain wavelength characteristics (specifically, see “Gain Equalization of Er-doped Fiber Amplifier using Etalon Filter” Takeda, et al., The Institute of Electronics, Information and Communication Engineers, 1995 Autumn General Meeting, B-759, and “Development of Etalon-type Gain Flattening Filter”, Mizuno et al., Furukawa Electric Review No. 105, January 2000).
Furthermore, in a wavelength equalizing apparatus described in Japanese Unexamined Patent Publication No. 11-162032, there is proposed a technique in which parameters of a plurality of optical filters each having periodic transmission wavelength characteristics are determined by Fourier series expansion of wavelength characteristics inverse to given wavelength characteristics, and these optical filters are serially connected to thereby compensate for the wavelength characteristics.
It is known that the gain wavelength characteristics of the above described EDFA are varied depending on the fluctuation of ambient temperature. In the erbium-doped optical fiber (EDF), further, the composition of the base material, particularly, variations in the concentration of additive such as aluminum (Al) may occur in the manufacturing process, and hence the gain wavelength characteristics of the EDFA may be varied depending on the concentration of Al. The gain wavelength characteristics of the EDFA are also varied when a control for maintaining an output light level constant (output fixing control: ALC) is being performed by changing the gain, in the case where an input light level is fluctuated.
It is desired that the transmission wavelength characteristics of the above described gain equalizer can be variably controlled, with respect to such changes of the gain wavelength characteristics of the EDFA. However, with the etalon-type optical filter used in the conventional gain equalizer, it is difficult to flexibly change the loss wavelength characteristics of the gain equalizer, corresponding to the change of the gain wavelength characteristics of the EDFA, since the transmission wavelength characteristics are fixed by the distance between the parallel reflection films constituting the Fabry-Perot interferometer, the reflectance of each reflection film or the like.
The present applicant has disclosed a variable optical filter utilizing the magneto-optical effect in Japanese Unexamined Patent Publication No. 11-212044, and has proposed a technique for constructing a gain equalizer or the like by connecting the above described variable optical filters in multi-stages, in Japanese Unexamined Patent Publication No. 12-137604. The variable optical filter used in this gain equalizer or the like is different from the above described etalon-type optical filter, but is an optical element in which the shape of the transmission wavelength characteristics which change periodically is made variable, by using a Faraday rotator. As the technique related to an optical element using the Faraday rotator, there are known other various techniques which are disclosed in Japanese Unexamined Patent Publications No. 61-279806, No. 11-14939, No. 9-21608, and No. 59-151065.
When such variable optical filters which use the Faraday rotator are connected in multiple stages to constitute a gain equalizer or the like, the above described Fourier series expansion method is effective as one method of designing the transmission wavelength characteristics of each variable optical filter. However, it is not so easy to realize a variable optical filter having transmission wavelength characteristics which are matched with the “sine” function of each term expanded by the Fourier series with high accuracy, only by using the magneto-optical effect.
Therefore, the present applicant has proposed a technique for providing a structure that operates as an etalon (Fabry-Perot interferometer) with a variable optical filter using a Faraday rotator, to make a periodic transmission wavelength characteristic variable (Japanese Patent Application No. 2002-70870). According to this prior application, it has become possible to change a transmission wavelength characteristic in an axial direction of transmissivity according to a Faraday rotation angle given to an optical element. Therefore, it becomes possible to realize a transmission wavelength characteristic matched with a gain wavelength characteristic of an EDFA with high accuracy. However, in the abovementioned prior application, although it is possible to change the periodic transmission wavelength characteristic in the axial direction of the transmissivity, it has not been possible to change a periodic characteristic thereof. In the case where matching of the transmission wavelength characteristic is required to be performed with higher accuracy, it is desirable to also make the periodic characteristic with respect to the wavelength variable.
In this specification, “equalization” means that undesirable transmission wavelength characteristics are compensated so that they become the same or approximately same characteristics as desirable transmission wavelength characteristics.